1. Field of the Invention
An apparatus consistent with the present invention relates to a projection system and, more particularly, to a projection system that can adjust color balance by adapting the areas of color bars using a spatial filter.
2. Description of the Related Art
Projection systems are classified into three-panel projection systems and single-panel projection systems according to the number of light values which control the on/off operation of light emitted from a high-output lamp on a pixel-by-pixel basis and form an image. Single-panel projection systems include a smaller optical device than three-panel projection systems, but have an optical efficiency equal to ⅓ of that of the three-panel projection systems. This is because they use a sequential method to separate a red beam R, a green beam G, and a blue beam B of white light. Hence, attempts to increase the optical efficiency of single-panel projection systems have been made.
In a general single-panel projection system, light irradiated from a white light source is separated into R, G, and B beams using color filters, and the three color beams are sequentially sent to a light valve. The light valve appropriately operates according to the sequence of the color beams received and creates images. As described above, the single-panel optical system sequentially uses color beams so that the light efficiency is reduced to ⅓ of that of a three-panel optical system. A scrolling method has been proposed to solve this problem. According to the color scrolling method, white light is separated into R, G, and B beams, and the three color beams are sent to different locations on a light valve. Further, since an image cannot be produced until all of R, G, and B beams for each pixel reach the light valve, R, G, and B color bars are moved at a constant speed in a particular method.
In a conventional single-panel scrolling projection system, as shown in FIG. 1, white light emitted from a light source 100 passes through first and second lens arrays 102 and 104, a polarizing beam splitter array 105, and a condenser lens 107, and is separated into R, G, and B beams by first through fourth dichroic filters 109, 112, 122, and 139. To be more specific, the red beam and the green beam, for example, are transmitted by the first dichroic filter 109 and travel along a first optical path L1, while the blue beam B is reflected by the first dichroic filter 109 and travels along a second optical path L2. The red beam R and the green beam G on the first optical path L1 are separated by the second dichroic filter 112. The second dichroic filter 112 transmits the red beam R along the first optical path L1 and reflects the green beam G along a third optical path L3.
As described above, the light emitted from the light source 100 is separated into the red beam R, the green beam G, and the blue beam B. The R, G, and B beams pass through first through third scrolling prisms 114, 135, and 142, respectively, thereby performing a scrolling operation. The first through third scrolling prisms 114, 135 and 142 are disposed on the first through third optical paths L1, L2, and L3, respectively, and rotate at a uniform speed such that R, G, and B color bars on a surface of a light valve 130 are scrolled. The green beam G and the blue beam B that travel along the second and third optical paths L2 and L3, respectively, are transmitted and reflected by the third dichroic filter 139, respectively, and then combined. Finally, the R, G, and B beams are combined by fourth dichroic filter 122. The combined beam is transmitted by a polarizing beam splitter 127 and forms an image using the light valve 130
The scrolling of the R, G, and B color bars due to rotation of the first through third scrolling prisms 114, 135, and 142 is shown in FIG. 2. Scrolling represents the movement of color bars formed on the surface of the light valve 130 when scrolling prisms corresponding to colors are synchronously rotated.
The light valve 130 processes image information according to an on/off signal for each pixel and forms an image. The formed image is magnified by a projecting lens (not shown) and projected onto a screen.
Since such a method is performed using an optical path provided for each color, an optical path correction lens must be provided for each color, and a component for re-collecting separated light beams must be provided for each color. Accordingly, an optical system is large, and yield is degraded due to a complicated manufacturing and assembling process. In addition, a large amount of noise is generated due to the driving of three motors for rotating the first through third scrolling prisms 114, 135, and 142, and the manufacturing costs of a conventional projection system adopting the above-described method is increased compared to a color wheel method adopting only a single motor.
In order to produce a color image using a scrolling technique, color bars as shown in FIG. 2 must be moved at a constant speed. The conventional projection system must synchronize a light valve with three scrolling prisms in order to achieve scrolling. However, it is not easy to control the synchronization. Further, because the scrolling prisms 114, 135, and 142 make circular motions, the color scrolling speed by the three scrolling prisms is irregular, consequently deteriorating the quality of an image.
The width of each of the color bars is determined according to the width of the beams traveling along the optical paths L1, L2, and L3. If the width of the beams traveling along the optical paths L1, L2, and L3 is narrow, the width of each of the color bars is narrow, and black bars K between the color bars are formed as shown in FIG. 3A. On the contrary, if the width of the beams traveling along the optical paths L1, L2, and L3 is wide, the width of each of the color bars is wide, and overlapping portions P of the color bars are generated as shown in FIG. 3B.
Such black bars K or overlapping portions P deteriorate the quality of a color image. This phenomenon may be explained using the etendue (E).
The etendue (E) denotes an optical conservation physical quantity in any optical system and is given by Equation 1:
                    E        =                              π            ⁢                                                  ⁢            A            ⁢                                                  ⁢                                          sin                2                            ⁡                              (                                  θ                                      1                    /                    2                                                  )                                              =                                    π              ⁢                                                          ⁢              A                                                      (                                  4                  ⁢                                      F                    /                    No                                                  )                            2                                                          (        1        )            wherein A denotes the area of an object whose etendue is to be measured, θ1/2 denotes half of an incident angle or an emitting angle of a light beam incident or emitted on the area A, and F/No denotes the F-number of lenses used in the optical system. The relationship equation,
  sin  ⁡      (                  θ                  1          /          2                    =              1                  (                      4            ⁢                          F              /              No                                )                      )  is obtained from Equation 1. According to Equation 1, the etendue (E) is determined by the area of the object and the incident angle of the incident beam or the F-number of lenses. The etendue (E) denotes a physical quantity that depends on the geometric structure of an optical system. The etendeu (E) at the starting point of the optical system must be equal to that at the ending point thereof in order to obtain an optimal light efficiency. That is, the etendue (E) must be conserved from the starting point to the ending point of the optical system. If the etendue at the starting point is less than that at the ending point, the area of the object A in Equation 1 is great when F/No is constant. On the contrary, if the etendue at the staring point is greater than that at the ending point, the area of the object A in Equation 1 is reduced so that light loss may be generated.
Here, when the starting point of the optical system is considered as a light source and the object is considered as a light valve, if the etendue (E) of the light source is greater than that of the optical system, the area of the color bars increases so that the colors are mixed at boundary portions between the color bars. On the contrary, if the etendue (E) of the light source is less than that of the optical system, the area of the color bars is reduced so that black bars K are generated between the color bars. The black bars K or the mix of the colors deteriorate the quality of a color image.
However, the black bars K need to be generated in a special case. For example, in a case where an LCD is used as the light valve 130, it may be difficult to sequentially process an image signal for each of the color bars. That is, when the color bars are scrolled sequentially, an image signal is changed according to the change of the color bars, making it difficult to sequentially process the changed image signal. In such a case, the black bars need to be generated between the color bars in order to produce time delay necessary for processing the changed image signal.
As described above, in the optical system which produces an image using the scrolling method, since the width of the color bars is required to be occasionally adjusted, means for adjusting the width of the color bars must be provided.